This invention relates to piezoelectric resonators and, specifically, to a method for optimizing the topology of a piezoelectric resonator-based network so that it may be implemented, either in monolithic or discrete form, with interconnects on one or either principal face of a piezoelectric plate or membrane.
A typical prior piezoelectric resonator comprises a wafer of piezoelectric material such as quartz or ceramic material provided with electrodes mounted on the wafer's opposing lateral surfaces Upon application of an alternating voltage to the electrodes, the piezoelectric material is driven electrically in a predetermined vibrational mode, for example, thickness shear, thickness extensional, etc., depending on the orientation or polarization of the piezoelectric material. The resonant frequency of the resonator is dependent on the overall wafer and electrode thickness and increases with a decrease in thickness. At high frequencies very thin wafers, plates, or films are required in order to have a half wavelength established across the thickness. These thin structures may be formed by thin film deposition onto a suitable substrate which leaves one principal surface of the material relatively inaccessible for interconnects. Resonators may be fabricated from these thin film and through different network configurations or with additional circuitry, such resonators can be combined to form filters or oscillators.
Prior resonators have been constructed in a number of ways. The most prevalent method is to construct individual resonators, mount and package them and then connect the packaged resonators into various circuit configurations. In an effort to reduce the overall circuit size, more than one resonator may be fabricated on a single plate of piezoelectric material and then interconnected on the plate to form a circuit. As shown in U.S. Pat. No. 3,222,622, the wafer may be cut to a desired thickness and the electrodes then mounted on opposing surfaces of the wafer. Another approach exemplified in U.S. Pat. No. 3,590,287 is to utilize a deposition process. The electrodes and piezoelectric material are deposited as metalization layers and a thin film, respectively, on a substrate such as a quartz wafer.
Although satisfactory resonators can be constructed with these techniques, they have their drawbacks. In both cases the two opposing electrodes of the resonator are on opposing lateral surfaces of the piezoelectric material and not coplanar. These electrodes, which connect the resonator to other circuitry such as integrated circuits or discrete components, may not be favorably positioned for making such connections. This is especially true if the piezoelectric material rests on a substrate and the electrode between the two materials is thus buried. Connecting the buried electrode to a discrete component such as a resistor is quite difficult. The situation is further complicated by the desirability of production testing as many as 600 piezoelectric resonator networks on each 4-inch-diameter wafer substrate prior to final assembly. Microwave probe testing requires all interconnect electrodes be on the top surface of the piezoelectric film.
One solution suggested in U.S. Pat. No. 3,222,622 is to provide a nonplanar conductive interconnect between a poorly positioned electrode and an additional electrode mounted in a more favorable position Connections to other circuitry may then be made from the additional electrode. As shown therein, an interconnect in the form of a discrete wire is added to a pi-network to connect an electrode mounted on the lower surface of the piezoelectric material to an electrode mounted on the upper surface of the material. The interconnect extends around the edge of the material, and is added as an additional step in the process of constructing the resonator.
An alternative solution might be to fabricate a via interconnect that extends through the piezoelectric material to make the desired connection. The interconnect is fabricated with additional steps in the process of constructing the resonator.
Neither of these solutions, however, is conducive to high volume, low cost manufacturing and testing by microelectronic techniques. Both add additional, costly steps to the manufacturing process. Within the microelectronic manufacturing context piezoelectric resonator networks must be designed and patterned to be compatible with manufacturing methods not employed within the conventional quartz crystal industry.